Anti-corrosive coating compositions

ABSTRACT

The invention relates to processes for coating metal surfaces and to corrosion resistant coating compositions for use in the processes. The compositions of the invention are solutions of a chromic acid salt of one or more organic bases having at least one straight or branched chain containing at least six carbon atoms in an organic solvent having a boiling point not greater than 250* C. at atmospheric pressure. The salts used are restricted to those which are sufficiently soluble in the solvent at 20* C. to provide a solution containing at least 0.2% CrO3 w/v in the solution. Preferred bases are primary, secondary or tertiary amines and quaternary ammonium bases. In a preferred embodiment the composition also contains a material capable of effecting reduction of at least a part of the hexavalent chromium in the composition. For particular applications in which the composition is applied to metals which have been pretreated, washed and not dried the composition also contains a surfactant and, if necessary, a solubilizing agent for the surfactant.

United States Patent [191 Vessey et al.

COMPOSITIONS [75] Inventors: Clifford A. Vessey; Geoffrey M. Gibson; Kenneth U. Holker, all of I-iarrogate, England [.7 3] Assignee: Associated Chemical Companies Limited [22] Filed: May 20, 1970 211 Appl. No.; 37,488

Related U.S. Application Data [63] Continuation of Ser. No. 628,310, April 4, 1967, abandoned. I i [30] Foreign Application Priority Data April 5, 1966 Great Britain ..l5,082/66 Sept. 29, 1966 Great Britain.... ..43,63l/66 Oct. 25, 1966 Great Britain.... ..47,875/66 Nov. 24, 1966 Great Britain... ..52,63l/66 [52] U.S. Cl. ..l48/6.2, 148/616, 148/627 [51] Int. Cl. ..C 23f 7/26 [58] Field of Search ..260/438 .5; 148/6.2, 6.27, 6.21, 14816.16; 252/390 [56] References Cited UNITED STATES PATENTS 1,523,741 l/1925 Yonkman ..l48/6.2 1,837,112. 12/1931 Chappell et al. ..148/6.2 1,946,152 2/1934 Edwards.... ..l48/6.2 X

ANTI-CORROSIVE COATING Tosterud ..l48/6.2

45 March 6, 1973 2,270,386 l/l942 Sloan ..148/6.2 X 2,796,369 6/1957 Chester et a1 ..148/6.2

' 2,480,754 8/1949 McCarthy ..260/438.5 3,376,143 4/1968 Barkman l48/6.2 X 2,911,332 11/1959 Schuster et al. ..148/6.2

FOREIGN PATENTS OR APPLICATIONS 1,447,004 6/1966 France ..148/6.2

Primary Examiner-Ralph S. Kendall Attorney-Jacobs & Jacobs 57 ABSTRACT The invention relates to processes for coating metal surfaces and to corrosion resistant coating composiare. restricted to those which are sufficiently soluble in the solvent at 20 C. to provide a solution containing at least 0.2% CrO w/v in the solution.

Preferred bases are primary, secondary or tertiary amines and quaternary ammonium bases.

In a preferred embodiment the composition also contains a material capable of effecting reduction of at least a part of the hexavalent chromium in the comosition. or particular applications in which the composition is applied to metals which have been pretreated, washed and not dried the composition also contains a surfactant and, if necessary, a solubilizing agent for the surfactant.

6 Claims, No Drawings ANTI-CORROSIVE COATING COMPOSITIONS This application is a continuation of application Ser. No. 628,310 filed Apr. 4, 1967, now abandoned.

BACKGROUND OF THE INVENTION The formation of corrosion resistant coatings on metal surfaces by application of a solution of a hexavalent chromium compound in water, followed by heating to dry the coating and to effect reduction of some of the hexavalent chromium to the trivalent state is well known. It is also known to include some trivalent chromium compounds in the initial treating solution and/or to include reducing agents in the solution which will promote the formation of trivalent chromium during the heating stage. The purpose of the trivalent chromium is to form a chromic chromate type of compound which has a sufficiently low water solubility to ensure that the hexavalent chromium inhibitor is not removed from the surface of the metal immediately on contact with water.

SUMMARY OF THE INVENTION We have found that improved corrosion resistant coatings can be obtained from a composition comprising a solution of a chromic acid salt of .one or more organic bases having at least one straight or branched chain containing at least six carbon atoms, in an organic solvent having a boiling point not greater than 250 C. at atmospheric pressure. The chromic acid salt should be sufficiently soluble in the solvent at C. to provide a solution Containing at least 0.2% w/v CrO (equivalent) in the solution. The solvent should either notreact at all, or react only very slowly with the chromic acid salt at temperatures of less than 30 C. Examples of such suitable solvents are ketones, hydrocarbons, halogen substituted hydrocarbons, certain nitrohydrocarbons, esters, ethers, nitriles and substituted acid amides and mixtures of any of these so]- vents. The preferred chromic acid salts are those which on shaking with water give a solution .containing not more than 1% CrO (equivalent) at 20 C.

The preferred organic bases are those which contain branched chains. The branched chains may consist solely of carbon atoms, or they may consist of nitrogen and carbon atoms.

Included amongst the preferred bases are the simple primary, secondary and tertiary amines, and quatemary ammonium bases, possessing at least one branchedchain in the molecule, and substituted guanidines, biguanides,- and s-triazines possessing at least one branched chain in the molecule. Two particularly preferred classes'of branched-chain bases are the tertiary alkyl primary amines, in'which the amino group is joined to a carbon atom which itself is attached to three alkyl groups and the'so-called beta-amines, which are derived from straight-chain saturated hydrocarbons by introducing an amino group onto the second carbon atom (the beta-carbon atom) of the chain;

Examples of. the preferred'and particularly preferred bases are I di-(Z-ethyl-n-hexyl) amine, tri-(3,5,5- trimethyl-n-hex'yl) amine, the mixture of tertiary alkyl primary amines containing from 12 to 14 carbon atoms known as Primene 8l-R (the word Prlmene being a Trade Mark), the mixture of tertiary alkyl primary amines containing from 18 to 22 carbon atoms known as Primene JM-T, the mixture (the word Armeen secondary-alkyl primary amines derived from Z-aminon-undecane in the form known as Armeen L.ll (the word Armeen being a Trade Mark), 2,4,6-tris-( 2-ethyln-hexylamino)-s-triazine, the mixed 1,5-dialkyl biguanides derived from Primene" 81-R or Primene JM- T, and the mixed, N, N, N"-tri-alkyl guanidines derived from Primene 8 l-R or Primene .lM-T. Examples of the chromic acid salts of these bases are the chromates, generally of the composition 8,, ll Cr O and the dichromates, generally of the composition 12.312620. (where B represents one molecule of a monoacidic base).

It will be understood that the invention includes all organic base/chromic acid salts which have the appropriate solubility characteristics as defined above and that such salts can be used according to the invention either singly or in admixture with each other or one another. It will also be understood that bases which are too easily oxidized by chromic acid, and salts which too easily decompose by internal oxidation-reduction reactions, are to be avoided.

Another useful class of organic base according to the invention is provided by the condensation products of ethylene diamine and diethylene triamine with carboxylic acids of formula R.C0.0H, R being a branched chain group where the branching has arisen through substitution of alkyl or cycloalkyl groups on one or more of the carbon atoms of the chain, or indeed, by actual link-up of such a substituent alkyl group with the chain toform a cycloalkyl group. An example of such a carboxylic acid is provided by 'the well-known class of naturally occurring petroleum carboxylic acids called naphthenic acids. Such acids of formula R.C0.0H will react with ethylene diamine and diethylene triamine of heating to give the basic mono amides liCO'ffiFlfl-iifl lfiflz and R.CO.HN.CH2.CH

,.NH.CH,.CH,.NH, respectively. On further heating,

these compounds undergo cyclodehydration to the corresponding imidazolines of formulas:

NH N-CHz-CHz-NH:

respectively. Both the two basic monoamides and the two imidazolines form chromic acid salts which are soluble in the above solvents. Examples of preferred bases of this class are the imidazoline derived from ethylene diamine and the naphthenic acid marketed by Shell Chemical Company Ltd. under the name of naphthenic acid NA 230/SP, the monoamide derived from ethylene diamine and naphthenic acid NA' 230/SP, and the imidazoline derived from diethylene triamine and naphthenic acid NA 230/SP.

We have found-that when solutions of the preferred chromic acid salts in sufficiently volatile organic solvents are applied to the surfaces of metals, and the solvent allowed to evaporate spontaneously at ordinary temperatures, the metallic surface acquires a fair degree of corrosion resistance. The volatility of the sol vent must be high enough to permit its evaporation from the coated surface in a reasonable length of time, but not so high that the solution is difficult to store and handle. In general a suitable solvent will havea vapor pressure at 20 C. within the range 3-200 mm. of mercury, the preferred range being 5-100 mm. of mercury.

We have further found, however, that if the coated metal is heated to vaporize the solvent, a better degree of corrosion resistance is obtained. In this process of curingthe coating, some decomposition of the organic chromium compound whilst in contact with the metal surface occurs, resulting in reduction of part of the hexavalent chromium to a lower valency state. While such a stage is known to be necessary with water-based systems in order to obtain a coating of sufficiently low solubility in water, the chromic acid salts applied to the metal in the present invention already possess the required low water solubility, and it is therefore rather surprising that the curing stage with the new inhibitors increases the degree of corrosion resistance obtained. For satisfactory curing it is preferable that the solvent should be sufficiently volatile to evaporate rapidly from the hot coated metal, because it is not normally convenient or desirable that the curing stage should take longer than a few minutes. Suitable solvents have vapor pressures greater than 200 mm. of mercury at the curing temperature. The preferred solvents have boiling points below thecuring temperature.

Coating obtained from solvent-soluble organic hexavalent chromium compounds subjected to a curing stage, which may vary from a few seconds duration at high temperatures to several minutes at temperatures as low as l C., offer considerably improved corrosion resistance as compared with the more conventional coating compositions consisting of aqueous solutions of hexavalent chromium compounds. In fact, the corrosion resistance obtained is usually comparable with that which can be obtained from aqueous solutions, only by electrolytic processes. The coatings are an ideal base for subsequent painting or lacquer'ing, and enable painted products of very high corrosion resistance to be obtained. The metals which can be treated by this type of composition include ferrous metals, zinc, aluminum and their alloys.

Although for some purposes useful coatings can be obtained by applying to the substrate the chromic acid salts in a solution in'a suitable solvent such as xylene and subsequently heating to obtain the required degree of reduction, we have found that it is necessary for most applications to includein the coating composition a substance which accelerates the process of reduction of some of the hexavalent chromium during the heating stage. Suitable substances are those which are soluble in the organic solvent, sufficiently easily oxidized on heating with the organic hexavalent chromium compounds during the curing stageanclv which do not produce deleterious oxidation products. The substance accelerating reduction must be such that a composition containing it is sufficiently stable to allow normal storage. Suitable substances include oxidizable materials such as linseed oil, castor oil, and fatty alcohols. Substances which, in addition to serving as reducing agents, are also film forming resins and/or polymers may also be used to assist curing. One example of such a material is the well known series of epoxy resins. The inclusion of these materials usually gives a coating with an even higher corrosion resistance than coatings obtained from compositions obtained using a reducing agent such as castor oil because the resin system progiving rise to products which are themselves reducing agents.

When solvents which are immiscible with water are being used, such as hydrocarbons and halogen substituted hydrocarbons, coatings can be obtained by applying the solution of the organic chromium compound emulsified with an aqueous solution of a mineral acid, such as nitric or sulphuric acids. However, when an emulsion type of system is employed, we have found it is advantageous to include in the aqueous phase chromic acid and trivalent chromium ions. As with the solvent solutions, it is possible to omit the mineral acid from the emulsion type system and rely solely on the curing period to effect the necessary reduction of the hexavalent chromium in the coatings.

The coating solutions or emulsions can be applied to the metal surfaces by conventional means, for example by spraying, brushing or dipping. As stated previously, the metals which can be coated by this type of composition include ferrous metals, zinc, aluminum and their alloys. in addition we 'have now found that coating compositions according to the invention may advantageously be employed to obtain improved corrosion resistance and better paint adhesion on surfaces which have already been treated by a conventional metal pretreatment process. Such pretreatment processes include electrolyticv chromating processes, phosphating processes and processes which involve the deposition of thin layers of other metals on to the substrate, for example the electrolytic deposition of metallic chromium for the purpose of conferring improved paint/lacquer adhesion to the substrate.

In cases where the new anticorrosive coating compositions are being applied as a final treatment to a substrate which has previously been pre-treated by another process, it is usually sufficient, in order to obtain the requisite corrosion resistance and lacquer-paint adhesion, to apply a very much more dilute solution or emulsion than is necessary for the same substrate with bare metal surfaces. Thus, as an after-treatment for phosphated steel a solution containing 1% CrO equivalent of the organic chromic acid salt would normally be employed, while for steel which has not previously been treated, a solution containing the equivalent of 5 percent of chromic acid would typically. be used.

Alternatively, it is possible to apply a very smallamount of a much more concentrated solution, for example, by electrostatic methods.

In certain cases, it is possible to apply the organic solution or emulsion containing the chromic acid salt to metal immediately after the pre-treatment process without an intermediate drying stage. Such dewatering" compositions are normally used in white spirit solution and contain as surfactants salts of alkylsubstituted aryl sulphonic acids or polyglycol ethers of alkyl phenols or fatty alcohols. It is frequently necessary to incorporate a solubilizing agent such as benzyl alcohol or tertiary butyl alcohol to dissolve these surfactants in white spirit solution. The surfactant and the solubilizing agent must be compatible with the chromic acid salt in the composition and must not react at all, or react only very slowly, with the chromic acid salt at temperatures less than 30 C. An example of such a surfactant is sodium dodecyl benzene sulphonate, and an example of such a solubilizing agent is t-butyl alcohol. The minimum effective concentration of the chromic acid salt is 0.04% w/v CrO equivalent for imparting good corrosion resistance. By use of a dewatering composition of this type, water adhering to the substrate surface is displaced by a simple dip in the chromic acid salt-containing treating solution, and the solvent is removed and the coating cured by stoving in the normal manner. In this way, it is possible to eliminate the necessity for washing the treated substrate with demineralized waterwhich is usually the final rinse in conventional pre-treatment processes, and at the same time obtain a product with improved corrosion resistance and paint/lacquer adhesion.

The invention is further illustrated by the following examples.

EXAMPLE 1 A 6 inch X 4 inch panel of mild steel was degreased and pickled in nitric acid and dried. The panel was then mounted in a Sheen Spinner and a quantity of a solution of thedichromate of Primene 8l-R in xylene containing 5% w/v CrO equivalent was-poured on to the center of the panel. After spinning, the solvent was allowed to evaporate at ambient temperature and the panel then exposed to neutral salt spray. After 7 hours it rated a mark of 3 on a 0 10 scale in which 0 no corrosion, and 10 100 percent corrosion. Untreated steel rated a mark of 8. A panel treated similarly but stoved at 200 C. for 3 minutes before exposure to salt spray, rated a markof 1 after 7 hours exposure.

EXAMPLE 2 A 6 inch X 4 inch. panel of mild steel was degreased and pickled in nitric acid and ,driedflhe panel was then coated as in Example 1 with a solution of the dichromate of Primene 81-R in'xylene containing 5% CrO equivalent, to which had been added 1 percent by volume of castor oil. After coating, the panel was stoved at 200 C for 1 minute.

The grey corrosion resistant surface obtained rated a marking of I after 24 hours exposure to salt spray.

EXAMPLE 3 EXAMPLE 4 A steel panel was prepared as in Example 1 and coated with a xylene solution of the dichromate of Primene 8l-R in xylene to which has been added 1% w/v of a xylene solution of the sulphate of Primene 81- R containing 5% S0 equivalent. After coating, the panel was stoved for 1 minute at 200 C. I

The grey corrosion resistant surface obtained rated a marking of 1 after 24 hours exposure to salt spray.

EXAMPLE 5 A steel panel was prepared as in Example 1 and coated with an emulsion prepared by mixing equal volumes of the following solutions A and B and shak- So lution A.

14.3 g. Primene 8 l-R dichromate. 4 g. Dodecyl benzene sulphonic acid.

25 g. of a nonyl phenol polyglycol ether containing 8 EXAMPLE 6 A steel panel was prepared as in Example 1. This was then coated with a trichloroethylene solution of Primene 8l-R dichromate containing 5% w/v CrO, Y

equivalent, and the coating cured by stoving the panel for 1 minute at 200 C.

The corrosion resistant surface rated a marking of 1 after 24 hours exposure to salt spray.

EXAMPLE 7 A steel panel was prepared as in Example 1 and coated with a methyl ethyl ketone solution of the dichromate of methylated Primene 8 l -R containing 5% w/v CrO equivalent. The coating was cured by stoving the panel for 2 minutes at 200 C.

The grey corrosion resistant surface rated a marking of I after 7 hours exposure to salt spray.

The methylated Primene 8l-R- from which this dichromate was prepared consisted of a mixture of Primene 8l-R with its monomethyl and dimethyl derivatives, and had an equivalent weight corresponding to that of the monomethyl derivative.

EXAMPLE 8 A steel panel was prepared as in Example 1 and coated with a nitrobenzene solution of Primene 8l-R dichromate containing 5% w/v CrO equivalent and the coating cured by stoving the panel for 1 minute at 200 C.

The corrosion resistant surface rated a marking of 3 after 7 hours exposure to salt spray.

EXAMPLE 9 A steel panel was prepared as in Example 1 and coated with a xylene solution of the dichromate of Primene 8 1-R containing 5% w/v CrO equivalent and 25% w/v of an epoxy resin of the type formed by condensation between epichlorhydrin and diphenylolpropane (such as the Epikote 815 manufactured by Shell Chemical Company). The coating was cured for three to 5 minutes at 200 C. I

The treated steel was untouched after 7 days exposure to salt spray.

EXAMPLE 10 Steel which had been cathodically treated in a solution containing chromate ions was dipped in a xylene solution of Primene 8l-R dichromate containing 1% w/v CrO equivalent and 0.2% w/v castor oil and heated at 200 C. for 1 minute. The resultant product was unattacked after exposure to neutral salt spray for 200 hours, whereas chromated steel without after treatment began to show corrosion after 40 hours.

EXAMPLE 1 1 Steel which had been phosphated according to conventional processes, washed with demineralized water and dried, was treated as described in Example 10.

Conventionally phosphated steel rated a marking of 7 after 7 hoursfexposure to salt spray. Phosphated steel, after treatment by organicdichromate, rated a marking of 3 after 7 hours and the paint/lacquer adhesion to the surface was also improved.

EXAMPLE 12 Steel which had been phosphated according to conventional processes and washed with tap water only and left undried'was treated with a xylene solution of Primene 8l-R dichromate containing 1% w/v CrO equivalent and 0.2% w/v of a dispersing agent consisting of equal weights of dodecyl benzene sulphonic acid and a nonyl phenol polyglycol ether containing 8 ethylene oxide units. The steel was then stoved for 1 minute at 200 C to give aproduct with similar corrosion resistance and paint/lacquer adhesion to that found in Example 1 1.

EXAMPLE l3 EXAMPLE 14 A clean 6 inch X 4 inch steel plate was treated with a xylene solution of 2,4,6-tris-(Z-ethyl-n-hexylamino)-striazme dichromate containing 5% w/v r 3 equivalent. After stoving for 3 minutes at 200 C. a corrosion resistant coating was obtained which formed an excellent base for subsequent lacquering or painting and which rated a marking of 1 after 24 hours exposure to salt spray.

EXAMPLE 15 A clean 6 inch X 4 inch steel plate was treated with a xylene solution of Armeen L1 1 dichromate containing 5% w/v CrO equivalent. After stoving for 1 minute at 200 C. a brown corrosion resistant coating was obtained, which was unattacked after 24 hours exposure to salt spray, and rated a marking of I after 48 hours.

What is claimed is:

l. A process for providing a corrosion resistant coating on metals which comprises applying to the metal surface a corrosion resistant coating composition for metals comprising a solution of a dichromate salt of at least one organic nitrogenous base having at least one straight or branched chain containing at least six carbon atoms in an organic solvent having a boiling point not greater than 250 C at atmospheric pressure and a reducing agent, the salt being sufficiently soluble in the solvent at 20 C to provide a solution containing-at least 0.2% CrO w/v in the solution.

2. A process as claimed in claim 1 in which the organic base is a primary, secondary or tertiary amine or a quaternary ammonium base.

3. A process as claimed in claim 2 wherein the coating is cured by heating to accelerate the evaporation of .the solvent and to bring about at least a partial reduction of the hexavalent chromium in the composition. I

4. A process for providing a corrosion resistant coating on metals which comprises applying to the metal surface a corrosion resistant coating composition for metals comprising a solution of a dichromate salt of at least one organic nitrogenous base having at least one straight or branched chain containing at least six carbon atoms in an organic solvent having a boiling point not greater than 250 C at atmospheric pressure, the salt being sufficiently soluble in the solvent at 20 C to provide a solution containing at least 0.2% CrO w/v in the solution also containing a material or materials which are capable of effecting reduction of at least a part of the hexavalent chromium in the composition.

5. A process as claimed in claim 4 wherein the reducing agent in the composition is an epoxy resinous material or materials which effect reduction of at least part of the hexavalent chromium in the composition after it has been applied to the metal surface.

6. A process for providing a corrosion resistant coating on metals which comprises applying to a metal surface a corrosion resistant coating composition for metals comprising a solution of a dichromate saltof at least one organic nitrogenous base having at least one straight or branched chain containing at least six carbon atoms in an organic solvent having a boiling point not greater than 250 C at atmospheric pressure and a reducing agent, the salt being sufficiently soluble in the solvent at 20 C to provide a solution containing at least 0.2% CrO w/v in the solution. 

1. A process for providing a corrosion resistant coating on metals which comprises applying to the metal surface a corrosion resistant coating composition for metals comprising a solution of a dichromate salt of at least one organic nitrogenous base having at least one straight or branched chain containing at least six carbon atoms in an organic solvent having a boiling point not greater than 250* C at atmospheric pressure and a reducing agent, the salt being sufficiently soluble in the solvent at 20* C to provide a solution containing at least 0.2% CrO3 w/v in the solution.
 2. A process as claimed in claim 1 in which the organic base is a primary, secondary or tertiary amine or a quaternary ammonium base.
 3. A process as claimed in claim 2 wherein the coating is cured by heating to accelerate the evaporation of the solvent and to bring about at least a partial reduction of the hexavalent chromium in the composition.
 4. A process for providing a corrosion resistant coating on metals which comprises applying to the metal surface a corrosion resistant coating composition for metals comprising a solution of a dichromate salt of at least one organic nitrogenous base having at least one straight or branched chain containing at least six carbon atoms in an organic solvent having a boiling point not greater than 250* C at atmospheric pressure, the salt being sufficiently soluble in the solvent at 20* C to provide a solution containing at least 0.2% CrO3 w/v in the solution also containing a material or materials which are capable of effecting reduction of at least a part of the hexavalent chromium in the composition.
 5. A process as claimed in claim 4 wherein the reducing agent in the composition is an epoxy resinous material or materials which effect reduction of at least part of the hexavalent chromium in the composition after it has been applied to the metal surface. 